1. Field of the Invention
This invention relates to apparatus and methods for generating or regenerating chemical substances, and more particularly to a method of recycling high purity hydrofluoric acid and hydrochloric acid. The method of this invention is particularly useful for point-of-use generation and recycling of high purity wafer cleaning chemicals.
2. Background Information.
Ultra large scale (ULSI) semiconductor devices manufactured today incorporate design line widths which are less than a micron, and dielectric films which are thinner than 300 angstroms. These features, which are expected to become even smaller over time, are extremely sensitive to contamination during their manufacture. As feature size decreases, the circuit devices, commonly referred to as chips or wafers, will become even more sensitive to contamination.
Contaminants which adversely effect ULSI devices come in various forms. Particles can cause open or shorted circuits and non-integral dielectric films. Organic and inorganic conlaminants, both metallic and nonmetallic, can result in poor film quality and decreased device performance and reliability. Metallic contaminants can be especially damaging. Taken as a whole, contaminants result in a large portion of device yield loss.
The processes presently used in the fabrication of ULSI devices rely heavily on chemical reagents, both gaseous and liquid. Wet cleaning solutions presently account for approximately 25% of the steps used in wafer manufacturing. In general, liquid reagents used in these processes contain much higher levels of contaminants than do gaseous reagents. In view of these contamination levels, the wafer manufacturing industry is attempting to decrease the reliance on liquid chemicals. However, gas phase cleaning procedures have proved difficult to develop, especially procedures for removal of particles and metallics. Hence, liquid based cleaning procedures are expected to be used for at least another decade. Thus reduction of contamination in these liquids is highly desirable. The industry predicts that tenfold reductions in contaminants every three to four years are required.
In a typical facility, relatively large volumes of liquefied chemicals are used in cleaning. An estimate of the volume of chemical usage by a large facility producing state of the art ULSI devices is as follows:
______________________________________ Chemical Type Usage (l./mo.) ______________________________________ 96% Sulfuric acid 30000-60000 30% Hydrogen peroxide 20000-40000 28% Ammonium hydroxide 10000-20000 37% Hydrochloric acid 5000-10000 49% Hydrofluoric acid 5000-10000 ______________________________________
These chemicals are highly toxic, highly corrosive and can pose environmental costs associated with their distribution, use and disposal. Large volumes of additional chemicals are required to neutralize the spent chemicals. The cost of treating the spent chemicals sometimes exceeds the cost of purchasing the chemical. Considerable cost reductions could be achieved if the chemical usage was reduced or if the chemicals were recycled.
In the past, various methods have been used or proposed to recycle liquids such as those listed above. Examples of such methods include carbon adsorption, reverse osmosis, ion exchange, membrane filtration, ultrafiltration and distillation However, these devices and methods have significant limitations and shortcomings.
Despite the need in the art for an a recycling method for cleaning chemicals such as aqueous hydrochloric acid (HCl), hydrofluoric acid (HF), hydrobromic acid (HBr) and nitric acid (HNO.sub.3), which overcomes the shortcomings and limitations of the prior art, none insofar as is known has been developed or proposed. Accordingly, it is an object of the present invention to provide a method of recycling HCl and HF used in various chemical processes which overcomes the limitations and shortcomings of the prior art. It is a particular object of this invention to provide a method which is useful for regenerating high purity hydrochloric acid and hydrofluoric acid used in the manufacture of ULSI circuits. Yet another object of this invention is to provide a method which is useable at the point of use of these cleaning chemicals, thus increasing manufacturing yields of ULSI circuits and reducing manufacturing costs.